1. Field of the Invention
This invention relates to apparatus of the type wherein an object is subjected to bursts of radiation the duration of each of which is short compared with the dwell time between successive bursts and the radiation as affected by the object is sensed for the purpose of deriving, for example, a signal representative of the nature of said object or merely a control signal for the activation of some utilization device or system. The invention is particularly related to analytical instruments such as fluorescence spectrometers in which the source for exciting fluorescence in the analytical sample is intended to operate in an intermittent or flash mode.
It is to be understood that, for the purposes of this description, an object may be said to affect the radiation falling thereupon if it acts on the radiation in any way, such as by re-directing it, re-distributing it or reemitting it at a different wavelength. In the case of a fluorescence spectrometer, for example, radiation in the UV or near UV region of the light spectrum would in fact be re-radiated by the object at a wavelength within the characteristic fluorescence spectrum of the object.
For literary ease and to facilitate a clear understanding of the invention, its general concept will be described in the context of a fluorescence spectrometer. This will not only enable the essential features to be readily appreciated but will also suggest the applicability of the invention to analogous situations, not necessarily in the context of material analysis.
2. Description of the Prior Art
In a known fluorescence spectrometer having a source of excitation radiation operating in the flash mode, the fluorescence of the sample substantially coincident with the duration of each excitation flash is analyzed by scanning it with a travelling continuous interference filter, at a slow enough rate of displacement relative to the flashing frequency to include a conveniently large number of flashes in one complete scan travel, the scan output being detected with a photomultiplier and finally integrated for presentation on a chart recorder. To mitigate the effect of radiation source fluctuations on the recorder output, a reference photomultiplier is used in addition to the sample photomultiplier and their respective outputs are ratioed. This scheme works well but cannot cope with the spurious responses introduced by unavoidable out-of-balances between the two outputs in terms of photomultiplier dark current characteristics, stray light, etc.
These out-of-balances are particularly significant in the prior art fluorescence spectrometers referred to, wherein the intermittency of excitation with periods of excitation very short in comparison intervals between flashes means that the signal content of each photomultiplier output is of a very short duration compared with the content due to the standing dark current between successive signals. The problem stems from a mode of operation that in other respects has been found most satisfactory, particularly in terms of instrument simplification. In the flash mode, instead of a radiation source continuously run at the high power levels of electrical energization required to ensure a continuum of radiation adequate for spectroscopic purposes, a compact gas-discharge lamp is used that is pulsed at high-peak power in bursts of a few microseconds. In order to ensure an acceptable lamp life, a comparatively long dwell period (typically some 20 milliseconds) must be tolerated between successive flashes; this exacerbates the problem.